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Oxidation of Refractory Benzothiazoles with PMS/ CuFe2O4: Kinetics and Transformation Intermediates Tao Zhang, Yin Chen, and TorOve Leiknes Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00701 • Publication Date (Web): 04 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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Environmental Science & Technology

1

Oxidation of Refractory Benzothiazoles with PMS/CuFe2O4:

2

Kinetics and Transformation Intermediates

3 4

Submitted by

5 6

Tao Zhang a, Yin Chen b, and TorOve Leiknes a*

7 8

a. Water Desalination and Reuse Center (WDRC), Biological and Environmental Sciences &

9

Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal

10

23966-6900, Kingdom of Saudi Arabia

11

b. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083,

12

People’s Republic of China

13 14 15

* Corresponding author: Tel.: + 966-12-8082193.

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E-mail address: [email protected]

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TOC Art

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ABSTRACT Benzothiazole

(BTH)

and

its

derivatives,

2-(methylthio)bezothiazole

(MTBT),

20

2-benzothiazolsulfonate (BTSA) and 2-hydroxybenzothiazole (OHBT), are refractory pollutants

21

ubiquitously existing in urban runoff at relatively high concentrations. Here, we report their

22

oxidation by CuFe2O4-activated peroxomonosulfate (PMS/CuFe2O4), focusing on kinetics and

23

transformation intermediates. These benzothiazoles can be efficiently degraded by this oxidation

24

process which is confirmed to generate mainly sulfate radicals (with negligible hydroxyl-radical

25

formation) under slightly acidic to neutral pH conditions. The molar exposure ratio of sulfate radical

26

to residual PMS (i.e. Rct) of this process is a constant which is related to reaction condition and can

27

be easily determined. Reaction rate constants of these benzothiazoles towards sulfate radical are (3.3

28

± 0.3) × 109, (1.4 ± 0.3) × 109, (1.5 ± 0.1) × 109 and (4.7 ± 0.5) × 109 M-1s-1, respectively (pH 7 and

29

20 oC). Based on Rct and these rate constants, their degradation in the presence of organic matter can

30

be well predicted. A number of transformation products were detected and tentatively identified

31

using triple-quadruple/linear ion trap MS/MS and high-resolution MS. It appears that sulfate radicals

32

attack BTH, MTBT and BTSA on their benzo ring via electron transfer, generating multiple

33

hydroxylated intermediates which are reactive towards common oxidants. For OHBT oxidation, it

34

prefers to break down the thiazole ring. Due to competitions of the transformation intermediates, a

35

minimum PMS/pollutant molar ratio of 10-20 is required for effective degradation. The flexible

36

PMS/CuFe2O4 could be a useful process to remove the benzothiazoles from low DOC waters like

37

urban runoff or polluted groundwater.

38



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39

INTRODUCTION

40

Benzothiazole derivatives are used in large volumes as vulcanization accelerators and are present

41

in all kinds of rubber-made products. Because of their wide application and various toxicity effects,

42

release of these compounds is of environmental concern.1-3 The parent molecules initially applied,

43

such as 2-morpholinothiobenzothiazole, however, are usually not detected in water. It is believed

44

they undergo quick transformations in the environment,4 and as such only breakdown products of the

45

parent compounds are frequently detected in urban runoff (e.g. stormwater), treated municipal

46

wastewater and surface waters. Typically four breakdown products are frequently detected, i.e.

47

benzothiazole (BTH), 2-(methylthio)bezothiazole (MTBT), 2-benzothiazolsulfonate (BTSA) and

48

2-hydroxybenzothiazole (OHBT).4-7

49

be 1 order of magnitude higher in urban runoff (tens of µgL-1) compared to treated municipal

50

wastewater (several µgL-1).5 This difference is probably due to the dissolution of tiny rubber

51

particles abraded from automobile tires on roads, making urban runoff a major source of these

52

benzothiazoles in aquatic environments.5,

53

abiotic transformation.5 They are not effectively removed in current wastewater/stormwater

54

treatment practices.9, 10 Due to their refractory nature, these compounds have also been found in

55

groundwater and tap water.11 Reports show that BTH and OHBT are frequently detected from

56

human urine at levels of ng L-1,12 which possibly is attributed to their existence in drinking water. It

57

is known that BTH, MTBT and OHBT have acute and chronic toxicity effects in the test with

58

Ceriodaphnia dubia,13 however, there are still no reports on the possible toxicity of BTSA.

Concentrations of these compounds are commonly found to

8

These benzothiazoles are relatively resistant toward

59

The presence of these benzothiazoles in drinking water has raised some healthy concerns. Their

60

removal with oxidation techniques has therefore been investigated, where ozonation, UV irradiation, 4 ACS Paragon Plus Environment

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61

and advanced oxidation processes (AOPs) generating hydroxyl radicals (e.g. photocatalytic

62

oxidation, O3/H2O2 and UV/H2O2) have been tested.14-18 Results of these studies clearly show that

63

hydroxyl radical is the major oxidant species responsible for effective degradation of benzothiazoles.

64

Unfortunately, their transformation products during hydroxyl radical oxidation are rarely reported,

65

making it difficult to comprehensively assess the effects of their oxidative degradation.

66

In recent years, degradation of refractory pollutants by sulfate radicals has attracted many interests

67

in both research and application. Sulfate radical has a reducing potential comparable to or even

68

higher than hydroxyl radical.19 It reacts with compounds normally via electron transfer, and can be

69

more efficient than hydroxyl radical in the degradation of some contaminants.20, 21 Sulfate radicals

70

can be produced from peroxymonosulfate (PMS) or peroxydisulfate (PDS) during activation with

71

metal ions,22, 23 metal oxides,24-27 alkaline,28 heat,29 UV irradiation,21, 30, 31 or phenols and quinones.32,

72

33

73

groups to efficiently activate PMS generating oxidative radicals at neutral pH while requiring no

74

additional chemicals or energy.34-36 The PMS/CuFe2O4 is more flexible than hydroxyl radical-based

75

AOPs which usually require ozone and/or UV, and thus could be preferential in the treatment of

76

urban runoff that occurs periodically. Under these conditions, application of ozone and UV is limited

77

by high equipment costs and low operation frequency.

Magnetically separable CuFe2O4 was recently developed in our group as well as in several other

78

We investigated the stability and the surface catalysis mechanism of CuFe2O4 for PMS activation

79

in our previous work.34 However, there is still no idea to determine the concentration distribution of

80

radical species for the application of PMS/CuFe2O4 oxidation, especially the transient concentration

81

of sulfate radicals which are of great interest to remove refractory pollutants. Since the sulfate

82

radical concentration cannot be determined, pollutant removal rates in various water matrices 5 ACS Paragon Plus Environment


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consequently cannot be predicted, which probably impedes practical application of this process as

84

well as other sulfate radical-based oxidation processes. Regarding the highly stable benzothiazoles

85

which are of great concern for the reuse of urban runoff for arid and semi-arid regions, they are

86

rarely studied with sulfate radical oxidation for removal efficiency, kinetics and mechanism. In this

87

work, we established a kinetic method to quantify the transient concentrations of sulfate radical

88

during the PMS/CuFe2O4 oxidation, determined reaction rate constants of the four benzothiazoles

89

(BTH, MTBT, BTSA and OHBT) with sulfate radical, tentatively identified their transformation

90

intermediates via LC/high-resolution MS and LC/MSn analysis, and proposed their degradation

91

pathway accordingly. The results will shed some light on how to determine transient sulfate radical

92

concentrations and predict pollutant removal rates for the PMS/CuFe2O4 process as well as for other

93

sulfate radical-based oxidation processes, and also on the effectiveness and mechanism of sulfate

94

radial reactions with highly stable benzo-heterocyclic compounds.

95

EXPERIMENTAL SECTION

96

Chemicals and materials. Benzothiazole (BTH; 96%), 2-(methylthio)benzothiazole

97

(MTBT; 97%), potassium salt of benzothiazolesulfonic acid (BTSA), 2-hydroxybenzothiazole

98

(OHBT;

99

2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS; ≥ 98%), peroxomonosulfate (PMS)

100

(Oxone, KHSO5·0.5KHSO4·0.5K2SO4), and peroxodisulfate (PDS; ≥ 98%) were purchased from

101

Sigma-Aldrich. Nitric acid (64 - 66%), sodium tetraborate (≥ 99.5%), and sodium nitrite (≥ 99%)

102

were purchased from the same company. Methanol (HPLC grade), acetonitrile (HPLC grade), water

103

for LC-MS, formic acid, and ammonium acetate were purchased from Fisher Scientific.

104

6-hydroxybenzothiazole (≥ 96%) and 2-nitrobenzenesulfonic acid (≥ 97%) as authentic references of

98%),

N,N-diethyl-m-toluamide

(DEET;

97%),

nitrobenzene

(NB;

≥

99%),


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105

oxidation products were purchased from TCI America. Information of the benzothiazoles and probe

106

compounds is given in Table 1. The ultrapure water for HPLC elution and experiments was

107

produced with a Milli-Q water system.

108

Table 1. Benzothiazoles and probe compounds used in this study. Molecular weight (Da)

logKow

pKa

BTH

135.18

2.01

1.216

2-Methylthiobenzothiazole

MTBT

181.27

3.15

1.2237

2-Hydroxybenzothiazole

OHBT

151.18

2.12

8.916

Benzothiazole-2-sulfonic acid

BTSA

214.23

−0.39

2.4-1.07

N,N-diethyl-m-toluamide

DEET

191.27

2.18

0.6738

-

123.11

1.85

-

Compound

Benzothiazole

Nitrobenzene

Abbreviation

Structure

109 110

Hydrophobic acid (HPOA) was extracted from Suwanee river water with XAD-8 resin. efOM was

111

extracted from the effluent of a wastewater treatment plant of Jeddah, Saudi Arabia, also with

112

XAD-8 resin. A street runoff was collected at November 23rd, 2014, on KAUST campus (University

113

Boulevard near Harbor Square) during the second precipitation of that month. It was immediately

114

filtered with 0.45 µm glass-fiber filters (Whatman), characterized (pH = 7.7, DOC = 3.1 mg L-1, 7 ACS Paragon Plus Environment


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UV254 = 0.13 (1 cm cell), alkalinity = 22.5 mg L-1 (CaCO3), chloride = 13 mg L-1, sulfate = 14 mg

116

L-1, and nitrate = 0.9 mg L-1), and stored at 4 oC before using.

117

The preparation of spinel CuFe2O4 particles was described in our previous work and showed in

118

detail in Text S1 of Supporting Information (SI).34 The spinel oxide particles have characteristic

119

BET surface area of 20.2 m2 g-1, average pore size of 30 nm, average particle size of 0.3 µm, pHpzc

120

(pH at which the surface is zero-charged) of 7.9, and saturated

121

(21 oC).

magnetization (M-H) of 24 emu g-1

122

Experimental procedure. Kinetic study. Experiments were conducted in a glass bottle

123

wrapped with aluminum foil. Pre-determined volumes of stock solutions of the compound and PMS

124

were injected into 200 mL Milli-Q water to get desired initial concentrations. Tetra-borate (10 mM)

125

rather than phosphate was used as a buffer in most of the reactions, because phosphate is a strong

126

ligand for transition metals. The solution was mechanically stirred during the reaction at a rotary

127

speed of 700 rpm (degradation rates of the bezothiazoles were leveled off at rotation speeds over 550

128

rpm under the experimental conditions, meaning that the influence of surface diffusion was

129

minimized) and room temperature (20 oC). The reaction was initiated by introducing CuFe2O4

130

particles into the solution. Samples taken at specific time intervals were filtered with 0.45 µm

131

glass-fiber syringe filters (Whatman). The filtration was confirmed to have no impact on the

132

concentrations of the target compounds and PMS. For the analysis of the target compounds, sodium

133

nitrite solution was immediately introduced into the filtrate to quench residual PMS. The nitrite was

134

not added to the samples for the analysis of residual PMS.


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135

Oxidation product study. Experiments were conducted in 50 mL amber glass bottles with Teflon

136

caps. Twenty mL of the solution containing 0.1 mM of the target compound, various concentrations

137

(0.1-2 mM) of PMS and 1.5 g L-1 of CuFe2O4 particle were introduced into the reaction bottles and

138

shaken at a rotation speed of 300 rpm. The PMS concentrations were periodically monitored till

139

complete consumption. Then, the suspensions were filtered through 0.45 µm glass-fiber syringe

140

filters (Whatman) for the analysis of target compounds and oxidation products. No reductant was

141

further applied.

142

Analysis. DEET, nitrobenzene, BTH, MTBT, BTSA and OHBT were quantified on a Waters

143

HPLC equipped with a Luna C-18 column (150 × 4.6 mm, 5 µm, Phenomenex) at UV wavelengths

144

of 266, 263, 252, 282, 266 and 241 nm, respectively. The column elution condition is shown in

145

Table S1 (SI). The concentration of PMS was analyzed through catalytic transformation of PMS into

146

sulfate radical which reacts instantly with ABTS generating ABTS+. In this method, 0.5 mL of

147

ATBS solution (20 mM), 0.2 mL of CoSO4 solution (20 mM), and 10 mL of diluted H2SO4 solution

148

(2%) were mixed with 1 mL of the water sample, and then the absorbance at 734 nm was measured

149

on a spectrometer (DR 500, Hatch). The oxidation of ABTS under this experimental condition was

150

completed within 2 minutes, enabling a kinetic study of PMS decomposition in this work. The

151

calibration curve used for the determination of PMS is shown in Figure S1 (SI).

152

For the kinetic study of benzothiazoles’ decomposition in the presence of natural organic

153

matter/effluent organic matter, LC-triple quadrupole/linear ion trap MS/MS (1260 Infinity HPLC,

154

Agilent; Qtrap 5500 mass spectrometer, AB Sciex) operating at multiple reaction monitoring (MRM)

155

mode was applied using the same Luna C-18 column. BTH, MTBT and OHBT were detected at ESI



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156

positive mode; BTSA was detected at ESI negative mode. Column elution conditions and MS

157

settings are shown in Table S2 and Text S2 (SI).

158

Full mass scan and MS2/MS3 fragmentation scans for the oxidation products (OPs) were

159

conducted with the same LC-Qtrap MS/MS under the same column elution condition. Accurate

160

masses of the OPs were determined on a LC-high resolution MS system (Surveyor HPLC, Thermal;

161

LTQ Orbitrap Velos mass spectrometer, Thermo) with the same column and elution condition. The

162

empirical formulae of these OPs were proposed with Xcalibur software (Thermo). MS settings are

163

shown in Text S2 (SI).

164

Full mass scan with the LC-Qtrap MS was applied to find potential OP peaks. MS2 fragmentation

165

of each potential OPs was conducted to find their characteristic fragments that can be applied for

166

MRM detection. Potential OPs in samples oxidized at PMS/compound molar ratios of 1, 2, 5, 8, 10,

167

12, 15 and 20 were detected with the LC-Qtrap MS/MS in MRM mode. OP screening was based on

168

1) peak area (obtained with MRM) variation with the PMS/compound molar ratio (the peak area of a

169

reasonable OP should increase or increase and then decrease with PMS dosage), and 2) whether their

170

empirical formulae as determined with LC-Orbitrap MS are reasonable (e.g. numbers of each

171

elements). For OPs confirmed with the above steps, MS3 scan of their major MS2 fragments were

172

conducted with the LC-Qtrap MS/MS to tentatively propose their molecular structures. Because

173

authentic standards are not available to chromatographically confirm most of the proposed structures,

174

Mass Frontier 5.1 software (HighChem), which can predict fragmentation patterns of a given

175

structure based on various known fragmentation mechanisms, was also applied to assist excluding

176

unreasonable isomer structures for the OPs.


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177

OP peaks of BTH and MTBT were much more intensive at ESI positive mode than negative mode.

178

Moreover, no new peaks other than those detected in positive mode were detected at negative mode.

179

So, positive mode ionization was applied in the analysis of OPs of BTH and MTBT. OPs of OHBT

180

and BTSA can only be detected at ESI negative mode.

181

RESULTS AND DISCUSSION

182

Effectiveness of PMS/CuFe2O4 for benzothiazoles. Figure 1 shows decline of the

183

four benzothiazoles during PMS/CuFe2O4 oxidation in pure water. The concentration of these

184

compounds declined quickly under this condition, achieving almost complete removal (i.e. below

185

detection limits of 0.01-0.02 µM on the HPLC-UV) within 2 minutes for BTH, OHBT and BTSA at

186

initial PMS/compound molar ratio of 40. For MTBT, its complete removal was also achieved within

187

2 minutes when the initial PMS/MTBT molar ratio was raised to 50. Neither PMS alone nor

188

CuFe2O4 alone can remove the benzothiazoles appreciably (not shown here). This result indicates

189

that the PMS/CuFe2O4 oxidation is effective for oxidative degradation of these benzothiazoles. A

190

further eight-cycle PMS/CuFe2O4 oxidation was conducted by reclaiming the CuFe2O4 particles

191

from the reaction solution and reusing them in the next reaction cycle (the experimental and results

192

were shown in Figure S2, SI). The removal of these benzothiazoles in 2 minutes reaction slightly

193

decreased as the CuFe2O4 was repeatedly reused, but the removal rates in 10 minutes of each

194

reaction cycle were nearly the same, indicating that the activity of the CuFe2O4 was durable for the

195

oxidation of these benzothiazoles.



1.0 BTH,

[PMS]o = 40 µM

OHBT, [PMS]o = 40 µM

0.8

C/Co

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BTSA, [PMS]o = 40 µM MTBT, [PMS]o = 40 µM

0.6

MTBT, [PMS]o = 50 µM

0.4 0.2 0.0 0

5

10

15

20

Reaction time (min) 196 197 198

Figure 1. PMS/CuFe2O4 oxidation of individual benzothiazoles. Conditions: dosage of each benzothiazoles = 1 µM; CuFe2O4 dosage = 200 mg L-1; unbuffered initial pH = 5.6; T = 20 oC.

199

Evaluation of sulfate radical generation within the PMS/CuFe2O4 process.

200

The PMS/CuFe2O4 process produces radicals through surface interactions of the catalyst with PMS

201

(eq 1-4).34 The sulfate radical reaction with OH- ( kSO4•- , OH − = 6.5 × 107 M -1s-1 ) also produces

202

hydroxyl radical in water (eq 5), which is more significant at higher pHs.31, 39, 40

203

know that sulfate radical is dominant in this process because pollutant oxidation was scavenged

204

more by ethanol (reactive towards both hydroxyl radical and sulfate radical) than by tert-butanol

205

(more reactive towards hydroxyl radical than towards sulfate radical).34 However, exact proportions

206

of these radical species still cannot be quantified. Determination of the concentrations of these

207

radical species is a prerequisite for kinetic studies.

208

≡ Cu(II) + HSO5- → ≡ Cu(II)--(HO)OSO3-

209

≡ Cu(II)--(HO)OSO3- → ≡ Cu(III)-- - OH + SO 4 •-

Presently, we

(1)

(2)


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210

≡ Cu(III)-- - OH + HSO 5- → ≡ Cu(II)-- • OOSO 3- + H 2 O

211

2 ≡ Cu(II) -- • OOSO3- → 2 ≡ Cu (II) + O 2 +2SO 4•-

212

SO4•- + OH - → •OH + SO4 2-

213

To quantify the radical species (i.e. sulfate radical and hydroxyl radical) generated in the

214

(3)

(4)

(5)

PMS/CuFe2O4 process, nitrobenzene (NB) ( kSO •- , NB ≤ 106 M -1s-1 ; kOH• , NB = 3.9 ×109 M -1s-1 )41,

42

4

215

and DEET ( kSO •- , DEET = (1.9 ± 0.1) ×109 M -1s-1 , pH 7 ; kOH• , DEET = 4.95 ×109 M -1s-1 )38,

43

were

4

216

applied as probes in the mixture. Decomposition of PMS, DEET, and NB at pHs between 6.0-8.0 are

217

shown in Figure S3 (SI). The decomposition rates of PMS and DEET were lower at higher pH,

218

because the repulsion between PMS (mainly in the HSO5- form) and the slightly negatively charged

219

CuFe2O4 surface (pHpzc = 7.9) reduces their interaction and the subsequent radical formation. The

220

decline of NB concentration during PMS/CuFe2O4 oxidation was observed to be the same as that of

221

evaporation caused by stirring (i.e. without PMS and CuFe2O4 in Figure S3C, SI), which means that

222

there was no observable oxidative removal of NB. Based on the reaction rate constants reported in

223

literature shown above, NB reaction with sulfate radicals is extremely slow compared to its reaction

224

with hydroxyl radical and the DEET-sulfate radical reaction. The NB degradation result here

225

indicates clearly that the hydroxyl radical yield is negligible in the PMS/CuFe2O4 oxidation, and the

226

DEET is oxidized by sulfate radicals. Therefore, the decomposition of DEET can be described with

227

eq 6 (integrated in eq 7). The Rct, a concept developed by Elovitz and von Gunten to correlate

228

hydroxyl radical generation with ozone decomposition during ozonation,44 is adopted here to

229

describe the exposure ratio of sulfate radical to PMS (eq 8). At a given CuFe2O4 dosage, the

230

decomposition rate of PMS follows a pseudo-first order (Figure S3A, SI), which can be described by 13 ACS Paragon Plus Environment


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231

eq 9, where k is the pseudo first order decomposition rate which can be obtained by plotting the

232

logarithm of normalized PMS residual against reaction time. Then, the exposure of sulfate radical

233

can be expressed as a function of residual PMS during oxidation (eq 10). The DEET residual can

234

thus be described with eq 11. d [DEET] = kSO • - , DEET ⋅ [SO 4• - ] ⋅ [DEET] 4 dt

235

-

236

ln

(6)

t

[DEET] = -kSO • - , DEET ⋅ ∫ [SO 4 • - ]dt 4 [DEET]o 0

(7)

t

237

∫ [SO4

Rct =

•-

]dt

(8)

0 t

∫

[PMS]dt

0

238

ln

[PMS] = -k ⋅ t [PMS]o

t

239

(9)

t

∫ [SO

•4

0

t

]dt = Rct ⋅ ∫ [PMS]dt = Rct ⋅ [PMS]o ⋅ ∫ e − kt dt 0

(10)

0

t

[DEET] = -Rct ⋅ kSO • - , DEET ⋅ [PMS]o ⋅ ∫ e − kt dt 4 [DEET]o 0

240

ln

241

Based on the decomposition results of PMS and DEET shown in Figure S3A and S3B (SI), linear

242

relationships were observed between

ln

(11)

[DEET] and the exposure of residual PMS, [DEET]o

t

243

[PMS]o ⋅ ∫ e − kt dt , at pHs between 6.0-8.0 and also in the presence of the organic matter, HPOA and 0

244

efOM (Figure 2 with original data in Figure 3S, SI). Results indicates that, under a given condition

245

(i.e. specified water matrix, PMS and CuFe2O4 dosages), the Rct value is a constant. A constant Rct

246

value under a specific reaction condition indicates that the transient concentration of sulfate radical 14 ACS Paragon Plus Environment

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247

is proportional to the residual PMS. This means that sulfate radical cannot be accumulated during the

248

PMS/CuFe2O4 oxidation, it should have been rapidly consumed by the organic/inorganic substances

249

and its self-combination reactions just like what happens to hydroxyl radical during ozonation.31, 45,

250

46

251

a simple way to quantify sulfate radical exposure during PMS/CuFe2O4 oxidation.

As the Rct value can be easily calculated by monitoring PMS and the probe compound, it provides

0.0

y = -21.792x - 0.0087 R² = 0.995

ln([DEET]/[DEET]o)

-0.5

pH 6.0 pH 7.0 pH 8.0 HPOA

efOM

y = -17.965x + 0.0135 R² = 0.991

-1.0 y = -56.418x + 0.0381 R² = 0.992

-1.5

-2.0 0.00

y = -140.33x + 0.1495 R² = 0.991

0.01

0.02

0.03

t

0.04

y = -26.507x + 0.052 R² = 0.99

0.05

0.06

0.07

[PMS]o ⋅ ∫ e dt (M ⋅ s) − kt

252

0

253 254 255 256 257

Figure 2. Relationship between DEET degradation and the exposure of residual PMS during PMS/CuFe2O4 oxidation. Conditions in absence of HPOA/efOM: [DEET]o = 1 µM; [PMS]o = 20 µM; CuFe2O4 dosage = 200 mg L-1; 10 mM tetraborate buffered; T = 20 oC. Conditions in presence of HPOA/efOM: HPOA/efOM = 2 mg DOC L-1; [DEET]o = 1 µM; [PMS]o = 100 µM; CuFe2O4 dosage = 500 mg L-1; 10 mM tetraborate buffered pH = 7; T = 20 oC.

258

Reaction rate constants of sulfate radical with benzothiazoles. The

259

competition kinetic method was applied to determine second-order rate constants of reactions

260

between sulfate radicals and benzothiazoles using DEET as probe compound. This method of rate

261

constant determination is applicable for the PMS/CuFe2O4 oxidation, because concentrations of

262

benzothiazoes and DEET even at 20 times lower than those applied in the kinetic study have no



Page 16 of 30

263

appreciable adsorption on the CuFe2O4 (Figure S4, SI), meaning these compounds will be degraded

264

in the solution, and surface adsorption-induced degradation can be excluded.

265

Based on the data shown in Figure S5 (SI), second order rate constants of sulfate radicals with

266

BTH, MTBT, BTSA and OHBT were calculated to be (3.3 ± 0.3) × 109, (1.4 ± 0.3) × 109, (1.5 ± 0.1)

267

× 109 and (4.7 ± 0.5) × 109 M-1s-1, respectively. It was reported that BTH and OHBT react with

268

hydroxyl radicals at rate constants of (3.9-8.6) × 109 and (4-5.1) × 109 M-1s-1, respectively.16 Sulfate

269

radical is thus comparable with hydroxyl radical for BTH and OHBT oxidation. Lutze et al. showed

270

that sulfate radical reacts slower than hydroxyl radical with humic acid and bicarbonate

271

( kSO

272

•4

, HA

= (6.6 ± 0.4) ×103 M -1s -1 ; kOH• , HA = (1.4 ± 0.2) ×104 M -1s-1 ; kSO •- , HCO - = 2.8 − 9.1 × 106 M -1s -1 ;

kOH • , HCO - = 1 × 107 M -1s -1 );

4

40

3

therefore, sulfate radicals could be more effective than hydroxyl radicals

3

273

for the oxidative removal of BTHs for some real water.

274

Prediction of BTHs degradation in the presence of organic matter.

275

Concentrations of residual BTHs during PMS/CuFe2O4 oxidation theoretically can be predicted with

276

eq 12 in the presence of organic matter which ubiquitously exist in water. The evolution of the

277

benzothiazoles in the presence of HPOA/efOM during PMS/CuFe2O4 oxidation is shown in Figure

278

3A and 3B. Based on the measured second-order reaction rate constants and the Rct values calculated

279

from eq. 11 with the decomposition data of PMS and DEET (shown in Figure S6 (SI)), the predicted

280

evolution of the concentrations of the benzothiazoles fit well (Figure 3). The degradation of the

281

benzothiazoles spiked into a real street runoff also fitted well with the model prediction (Figure 3C)

282

(PMS and DEET decomposition in the street runoff were shown in Figure S6 (SI)). The result also

283

indicates that a PMS dosage over 200 µM is needed for a real street runoff if the less reactive MTBT

284

and BTSA are of concern for their concentration. 16 ACS Paragon Plus Environment

Page 17 of 30

Environmental Science & Technology t

285

[BTHs] = [BTHs]o ⋅ e

- Rct ⋅k

SO4• - ,BTHs

∫

⋅[PMS]o ⋅ e − kt dt

(12)

0

Concentration (µM)

0.12

A BTH MTBT OHBT BTSA

0.08

0.04

0.00 0

10

20 30 40 50 Reaction time (min)

60

286

0.15

Concentration (µM)

B BTH MTBT OHBT BTSA

0.10

0.05

0.00 0 287

10

20

30

40

50

60

Reaction time (min)



Concentration (µM)

0.12

Page 18 of 30

C BTH MTBT OHBT BTSA

0.08

0.04

0.00 0

5

10 15 20 25 Reaction time (min)

30

288 289 290 291 292 293 294

Figure 3. Degradation of benzothiazoles in HPOA (A) and efOM (B) solutions and in a street runoff (C): symbols represent measured data, and lines are predicted with the kinetic model. Conditions: PMS dose was 100 µM for HPOA and efOM solutions and 200 µM for the street runoff; DOC of HPOA/efOM solutions = 2 mg L-1; CuFe2O4 dosage = 200 mg L-1; HPOA and efOM solutions were buffered to pH 7.0 with 10 mM tetra-borate; T = 20 oC. Error bars represent standard deviation of three replicates.

295

Transformation products of benzothiazoles formed from PMS/CuFe2O4

296

oxidation. Oxidation products detected for BTH, MTBT, BTSA and OHBT are listed in Table

297

S3-S6 (SI). Evolution of peak areas of these products (detected with MRM mode) and the

298

benzothiazoles with initial PMS/compound molar ratio are presented in Figure S7-S10 (SI). Since

299

authentic standards for most of these oxidation products are not available, their structures were

300

proposed based on their empirical formulae (Table S3-S6, SI) and MS2/MS3 fragmentation patterns

301

as summarized in Table S7-S10 (SI). The Mass Frontier 5.1 software was also used to assist in

302

interpreting fragmentation patterns of the parent benzothiazoles and screening possible structures of

303

the oxidation products.


Page 19 of 30


304

1. Oxidation products of BTH. The fragmentation pattern of BTH provides a basis to interpret the

305

MS2 and MS3 results of the twelve oxidation products. For example, the loss of 27 amu in MS2

306

fragmentation is attributed to the detachment of CHN from the mother molecule (Table S7, SI).

307

Therefore, a 27 amu loss in the fragmentation of an oxidation product signifies that the -CH=N-

308

structure of the thiazole ring is not modified in the oxidation. Consequently, the loss of 44 amu

309

indicates the presence of intact C-S moiety.

310

Four isomers of OP 152 were observed with nearly the same MS2/MS3 fragmentation pattern

311

(Table S7, SI). According to their accurate masses (Table S3, SI), they have one more O atom than

312

the parent BTH. A loss of water (18 amu) was observed during MS2 fragmentation of OP 152, which

313

signifies the presence of a hydroxyl group next to an extractable hydrogen.47 Since losses of 27

314

(CHN) and 44 (CS) amu were observed respectively in the fragmentation of OP 152 and its

315

fragments, the extra O atom should not be present on the thiazole ring but on the benzo ring. The

316

retention time of 2-hydroxyl benzothiazole (OHBT) which has one O atom attached on 2-C of the

317

thiazole ring (16.2 min) didn’t match with those of OP 152 under the same column elution

318

conditions (Figure S11, SI). Moreover, there was no direct loss of 27 amu (CHN) from OHBT in

319

MS2 fragmentation (i.e. absence of m/z 125 (Fig S12, SI)), which also confirms that the

320

hydroxy-benzo structure of OP 152 is reasonable. Only one hydroxylated benzothiazole with OH

321

substitution on the benzo ring was commercially available, i.e. 6-hydroxybenzothiazole. It has the

322

same retention time (13.9 min) and MS2 fragmentation pattern with OP 152-3, meaning that the

323

structure of this product can be confirmed.

324

OP 168 of BTH has three isomers with two more O atoms than the parent BTH (Table S3, SI).

325

Simultaneous hydroxylation on C atoms of the thiazole ring and the benzo ring seems impossible, 19 ACS Paragon Plus Environment


Page 20 of 30

326

because sulfate radical-oxidized OHBT sample had no peaks matching the oxidation products of

327

BTH (Figure S11, SI). Losses of 44 (CS) and 27 (CHN) amu were observed in further fragmentation

328

of its fragments m/z 150 (loss of H2O) and 140 (loss of CO), respectively (Table S7, SI), indicating

329

that the two O atoms present on the benzo ring but not on 1-S or 3-N of the thiazole ring. A broken

330

benzo ring having two side aldehyde groups is not a possible structure for OP 168, because with

331

fragmentation prediction of Mass Frontier 5.1: 1) this structure has no loss of 18 amu (H2O) which

332

was observed for OP 168, and 2) it has a loss of 16 amu (O) due to charge remote rearrangement

333

which was not observed for OP 168. OP 184 has three isomers with three more O atoms than the

334

parent BTH. Likewise, the broken benzo ring having olefin alcoholic aldehyde or olefin carboxylic

335

aldehyde groups are not likely structures for OP 184. Purpald test which is sensitive to aldehydes

336

was also applied for the water samples (procedures shown in Text S3, SI), and no aldehyde

337

formation can be noticed. Substitution of three hydroxyl groups on the benzo ring of BTH was thus

338

proposed for OP 184. OP 182 with two H atoms less than OP184 seems to have a quinone moiety.

339

The loss of 44 amu (CS) during MS2 fragmentation of OP182 as well as the loss of 27 amu (CHN)

340

during MS3 fragmentation of its fragment m/z 110 indicates that the three O atoms are present on the

341

benzo ring. The losses of 44 and 27 amu were also observed during MS3 fragmentation of fragments

342

m/z 182 and 110 respectively for OP 200 which has four O atoms more than the parent BTH. The

343

substitution of four hydroxyl groups on the benzene ring of BTH was thus proposed as the structure

344

of OP 200. Further oxidation of these hydroxylated products by sulfate radicals likely leads to

345

opening of the aromatic ring forming small carboxylates (as suggested in sulfate radical oxidation of

346

phenol, chlorophenol, and quinone).48, 49 However, these potential small carboxylate products were


Page 21 of 30


347

not detected with LC-MS neither in positive nor in negative mode, possibly because they are very

348

hydrophilic and quickly eluted from the column under the analytical condition.

349

Most of the oxidation products like OP 152, OP 168, OP 182 and OP 200 are still quite reactive

350

towards sulfate radicals (Figure S7, SI). It was confirmed in our previous work that radicals can be

351

generated from PMS at a molar ratio of 1:1 in the PMS/CuFe2O4 process.34 Because of competitive

352

consumption of sulfate radicals by these oxidation products, here a PMS/BTH stoichiometry over 12

353

is required to degrade over 90% of BTH under the experimental conditions. Further experiment was

354

conducted to see whether these oxidation products can be oxidized by cheaper oxidants like

355

peroxodisulfate (PDS) ($0.74 per kg vs. $2.2 per kg of PMS) and permanganate (PM) ($1.5 per kg)

356

which readily react with hydroxylated aromatic structures.32,

357

introduced into the samples having been oxidized by PMS/CuFe2O4 at PMS/BTH = 8 and filtered

358

with 0.45 µm filters (i.e. in the absence of CuFe2O4 particles during PDS or PM oxidation). Dosages

359

of PDS and PM were twice of the initial BTH. In this case, the molar ratio of total oxidant to BTH

360

will be comparable with that of PMS/BTH = 10. Figure S13 (SI) shows peak areas of the products

361

obtained from PMS/CuFe2O4 oxidation at PMS/BTH = 10 and those further oxidized by PDS and

362

PM after PMS/CuFe2O4 oxidation at PMS/BTH = 8. It is clear that PDS and PM can destroy these

363

oxidation products more effectively than sulfate radicals, which means that most of the products can

364

be selectively oxidized by the common oxidants possibly because of their reactive hydroxy-benzo

365

structures. This result also suggests that a combination of sulfate radicals and common oxidants

366

would be able to improve the efficiency and reduce the cost of oxidation. The combination could

367

also possibly be useful to reduce the formation of toxic products when chloride is present at high

50

PDS and PM were individually



Page 22 of 30

368

concentrations in the water, as chlorine radicals (formed by sulfate radical oxidation of chloride)

369

readily react with phenolic structures forming chlorinated products.48, 49

370

2. Oxidation products of MTBT. MTBT has losses of 15 (CH3), 32 (S), 44 (CS), 27 (CHN), and 58

371

(CNS) amu during its MS2 and MS3 fragmentation (Table S8, SI). OP 198, having one more O atom

372

than the parent molecule (Table S4, SI), lost 15, 27, 32, and 44 amu during fragmentation, which

373

means that the thiazole ring and its -S-CH3 side chain were not modified (i.e. hydroxylation occurred

374

on the benzo ring). The same fragmentation losses were observed for OP 214 which has two isomers

375

and two more O atoms than MTBT. Two OH-group substitutions on the benzo ring was thus

376

proposed for OP 214. Opening of the benzo ring with formation of two side aldehyde groups was

377

excluded because of the same reason as mentioned for OP 168 of BTH. OP 230 has three isomers

378

and three more O atoms than MTBT. The losses of 15, 27, 32, and 44 amu were observed during its

379

MS2 and MS3 fragmentations, indicating that the thiazole ring is intact. It should have three OH

380

groups substituted on the benzo side. Likewise, OP 246 should have four hydroxyl groups

381

substituted on the benzo ring of MTBT, as it has four more O atoms and losses of 15, 32, 44, and 58

382

amu during MS2 and MS3 fragmentation. A stoichiometry of PMS/MTBT higher than 20 is required

383

to achieve over 90% degradation of MTBT (Figure S8, SI). The higher oxidant dose required than

384

BTH oxidation indicates a higher consumption of sulfate radicals by the transformation products of

385

MTBT.

386

3. Oxidation products of BTSA. BTSA has losses of 64 (SO2) and 28 (CO) amu during MS2 and

387

MS3 fragmentation respectively at ESI negative mode (Table S9, SI). Successive losses of 80 (SO3)

388

and 76 (C6H4) amu were also observed. The losses of 64, 28 and 80 amu were observed during the

389

fragmentation of OP 230, indicating that the sulfonate group was retained. No loss of 76 amu means 22 ACS Paragon Plus Environment

Page 23 of 30


390

that the benzo ring was modified. The extra O atom of OP 230 compared to BTSA (Table S5, SI)

391

should be present on the benzo ring. Accordingly, di- and tri- hydroxyl substitution on the benzo

392

ring were proposed for OP 246 and OP 262, respectively. OP 260, having two H atoms less than OP

393

262, probably has a hydroxylated quinone ring.

394

4. Oxidation products of OHBT. Losses of 29 (CHO), 43 (CHON), 60 (COS) and 69 (C3H3ON)

395

amu were observed for OHBT during negative mode MS2 fragmentation (Table S10, SI). OP 172,

396

which is one C less and two H and two O more than OHBT (Table S6, SI), has a loss of 64 amu

397

which is characteristic for the sulfonate group as shown in the fragmentation of BTSA (Table S9, SI).

398

A loss of 92 amu (C6H6N) was observed, which means that the benzene ring is retained. None of the

399

characteristic fragmentation losses of OHBT was observed for OP 172, indicating a breakage of the

400

structure. Therefore, OP 172 should have a sulfonate group attached on the benzene ring, and the

401

thiazole ring is damaged. OP 202 is two H less and two O more than the OP 172. Since losses of 64,

402

80, and 46 (NO2) amu were observed, the OP 202 should have a sulfonate group and a nitro group

403

attached on the benzene ring, which is reasonable as its retention time and fragmentation pattern

404

matches well with that of 2-nitrobenzenesulfonate. At PMS/OHBT molar ratio of 20, the OHBT

405

dosed (100 µM) as well as the OP 172 were nearly completely degraded. In the meantime, the

406

2-nitrobenzenesulfonate produced was determined to be 6.1 µM, which is much lower than the

407

OHBT dosed, suggesting that this product could be also degradable during the PMS/CuFe2O4

408

oxidation. The degradation of 2-nitrobenzenesulfonate by PMS/CuFe2O4 was confirmed as shown in

409

Figure S14 (SI). Its degradation rate was lower than that of OHBT, which could be the reason for its

410

accumulation within the PMS dosages applied here.



Page 24 of 30

411

Proposed reaction mechanism. According to the structures proposed for the oxidation

412

products, we tentatively conclude that sulfate radicals preferentially attack the benzo ring of BTH,

413

MTBT, and BTSA. For OHBT, sulfate radical attacks more readily the thiazole ring side than the

414

benzo ring.

415

Sulfate radical is a strong electrophile. It attacks organic compounds more preferentially via

416

electron transfer than hydrogen abstraction and addition.19, 40 Its reaction with benzene via electron

417

transfer is believed to produce hydroxycyclohexadienyl radical through rapid reaction of the benzene

418

radical cation with water.19, 51-53 It is known that hydroxycyclohexadienyl radical can efficiently

419

react with oxygen forming peroxy radical which further decomposes giving phenol as product.54 The

420

study of Anipsitakis et al. shows that sulfate radical reaction with phenol further produces dihydroxy

421

products (catechol and hydroquinone) following the same mechanism.48 It seems that the sulfate

422

radical reaction with BTH, MTBT and BTSA follows a similar pathway with that of benzene and

423

phenol oxidation (Scheme 1). Once the hydroxylated products are formed, further attack on the

424

electron-enriched benzo ring via electron transfer is more favored, which leads to the formation of

425

products with multiple hydroxyl groups on the benzo ring side of these benzothiazoles. This is also

426

supported by the relatively high stoichiometric ratios of PMS required for effective removal of the

427

parent benzothiazoles. Besides hydroxylated products, quinone-like products were also produced,

428

which possibly proceeds through hydrogen abstraction of the hydroxylated products by either sulfate

429

radical or PMS itself.


Page 25 of 30


4

SO MS P ;O O H2 2

430 431 432

Scheme 1. Proposed degradation pathway of BTH, MTBT and BTSA during PMS/CuFe2O4 oxidation.

433

The sulfate radical oxidation of OHBT follows a different reaction pathway. The OH group on the

434

thiazole ring is more likely to be attacked by sulfate radicals than the benzo ring via electron transfer

435

and produces –O radical cation. The positive charge can shift to the 2-C due to the conjugate

436

structure. Further hydrolysis in water leads to the detachment of 2-C and the formation of a thiol and

437

a primary amine group (Scheme 2). The thiol group can be easily oxidized by sulfate radical into

438

sulfonate group, it can even be oxidized by PMS itself.55 The amine group is finally oxidized into a

439

nitro group. The formation of a nitro derivative from sulfamethoxazole which has an aniline moiety

440

was also reported in reaction with sulfate radicals generated with PMS/Co2+.56

441 442

Scheme 2. Proposed degradation pathway of OHBT during PMS/CuFe2O4 oxidation.

443

Environmental significance. In arid and semi-arid regions, urban runoff is considered a

444

potential water resource for aquifer recharge or nonpotable reuse after proper treatment.57

445

Benzothiazoles usually exist in urban runoff at relatively high concentrations. Effective removal of 25 ACS Paragon Plus Environment


Page 26 of 30

446

them is necessary before aquifer recharge to avoid groundwater pollution. The PMS/CuFe2O4 could

447

be a useful oxidation process in this case where the application of other advanced oxidation

448

processes (e.g. O3/H2O2 and UV/H2O2) has more limitations in adaptation to variable precipitation

449

and infrequent operation. Because of its high stability, the CuFe2O4 can also be fabricated into

450

paving bricks or filtration media for in-situ oxidative treatment of street runoff.

451

This study shows that the removal rates of benzothiazoles during PMS/CuFe2O4 oxidation can be

452

predicted with predetermined sulfate radical/PMS exposure ratio. This sulfate radical quantification

453

approach will also be useful in investigating the degradation of other refractory pollutants within this

454

process or other sulfate radical-related oxidation processes. Sulfate radical attack produces multiple

455

hydroxylated intermediates for benzothiazoles (except for OHBT). These hydroxylated products

456

seem readily degradable by cheaper oxidants like peroxydisulfate and permanganate. The

457

combination of sulfate radicals with these common oxidants could be practical and useful to reduce

458

treatment costs and improve the degradation of transformation intermediates.

459

ASSOCIATED CONTENT

460

Supporting Information. Ten tables, fourteen figures and three texts are included in the

461

supporting information. This information is available free of charge via the Internet at

462

http://pubs.acs.org.

463

ACKNOWLEDGEMENTS

464

This research reported in this publication was supported by funding from King Abdullah

465

University of Science and Technology (KAUST). We thank Prof. Jean-Philippe Croué (Curtin

466

University, Australia) for the gift of HOPA and efOM. The assistance of Ms. Tong Zhan and Dr. 26 ACS Paragon Plus Environment

Page 27 of 30


467

Julien Le Roux (WDRC, KAUST) and Mr. Salim Sioud (Analytical Core Lab, KAUST) in MS

468

analysis is gratefully acknowledged. We also appreciate the anonymous reviewers for their revision

469

suggestions which significantly improved the quality of this work.

470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505

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